1. Field of the Invention
The present invention relates to a penetrameter for use in the x-ray analysis of a broad range of different materials as well as to a method for calibrating the resolution of an x-ray inspection system used to conduct non-destructive evaluation of such materials.
2. Background Discussion
Radiographic and radioscopic data acquisition systems, are useful tools in the non-destructive evaluation (NDE) of both metallic and non-metallic materials using x-ray analysis. Flaws detectable using x-ray analysis include, but are not limited to, corrosion, material density anomalies, useful in revealing subtle low-density inclusions and voids in advanced composite material parts, and fiber discontinuity in composite materials.
FIG. 1 illustrates the basic principles of operation of a radiographic data acquisition system, generally indicated at 11, which typically employs an x-ray source 13 to provide a beam of x-rays 15 that impacts an object or part 17 to be analyzed. A detector 19, in the form of a photographic plate, is positioned to receive the x-rays that pass through and are scattered by the part 17. After exposure of the photographic plate to the x-rays, the plate is developed to form a shadow image or radiograph of the part 17. The radiograph is then use to conduct an analysis of the material structure of the part 17.
Referring to FIG. 2, one illustration of a radioscopic data acquisition system 21 is provided wherein the x-ray source 23 of the system 21 sends out a beam of x-rays 25 that passes through a specimen or part 27. The beam of x-rays 25 is then detected by an intensifier 29, connected to a CCD camera 31 which, in this case, is connected at its output to a image processor 33 that makes available an image which is displayed on a video monitor 35.
Thus, a radioscopic system is a non-film x-ray imaging system whereas the radiographic system is a film x-ray imaging system. The primary advantages of radioscopy over radiography is speed of operation and the ability to examine outsized, multi-configured structures.
A standard test piece is usually included in radiography and radioscopy systems as a check on the adequacy of the associated technique. The test piece is commonly referred to as a penetrameter in North America and an Image Quality Indicator (IQI) in Europe. Although applicable to both radioscopic and radiographic analysis, for the sake of simplicity, the following discussion will be limited to radiographic applications.
The penetrameter is typically a simple geometric form made of the same material as, or a material similar to, the specimen being radiographed. It includes some small structures (holes, wires, etc.) and is provided with a given thickness, the dimensions of both of which can bear some numerical relation to the thickness of the part being tested. This relationship is typically denoted in terms of some percentage value, i.e., the thickness of the penetrameter or diameter of the holes being a percentage of the thickness of the part being imaged.
In use the penetrameter is placed at the part and imaged therewith. Then, the smallest feature of the penetrameter visible on the resulting radiograph is determined, the quality level of the radiograph being determined thereby. For example, if the penetrameter has a thickness that is 2% of the thickness of the part and a hole therein has a diameter that is 1% of the thickness of the part and only the outline of the penetrameter is visible in the radiograph and not the hole, then the quality level is typically designed as being 2%.
The image of the penetrameter on the radiograph is permanent evidence that the radiographic examination was conducted under proper conditions. The use of penetrameters is advisable because they provide an effective check on the overall quality of the radiographic inspection.
Common penetrameters consist of a small rectangular piece of metal 39, as seen in FIG. 3A, or a circular piece of metal 41, as seen in FIG. 3B, containing several (usually three) holes 43, the diameters of which are related to the thickness of the penetrameter. Another penetrameter design also used, as best seen in FIG. 3C, is the German DIN (Deutsche Industrie-Norm) penetrameter which consists of a number of parallel metal wires 47 of various diameters sealed in a plastic envelope 45. The image quality is indicated by the thinnest wire visible on the radiograph. The DIN penetrameter system is such that only three penetrameters, each containing seven wires, can cover a wide variety of specimen thickness.
The hole type penetrameter, as seen in FIGS. 3A and 3B, is essentially a go, no-go gauge which indicates whether a specified quality level or resolution level has been attained, but in most cases does not indicate whether the requirements have been exceeded, or by how much. The DIN penetrameter, as seen in FIG. 3C, on the other hand is a series of seven penetrameters in a single unit. As such, the DIN penetrameter has the advantage that the radiographic quality level achieved can often be read directly from the process radiograph.
However, the DIN penetrameter is typically used in connection which metal parts which do not include fibers as are commonly included in composite parts. As a result of the parallel configuration of the metal wires of the DIN penetrameter, these metal wires are often difficult to distinguish from the fibers of the composite material which are also typically oriented in a parallel configuration.
As described above, in use, the penetrameter is placed with the specimen and a radiograph of the specimen and penetrameter is made. By examining the radiograph of the penetrameter to determine the smallest sized structure visible, it is possible to determine and evaluate the resolution level of the radiograph.
A recent development in the field of NDE using x-ray analysis is the use of low kilovoltage radiography and radioscopy which is a specific x-ray technology employing very low X-radiation (generally between about 7 and 30 kilovolts) while increasing the x-ray tube filament milliamperage. Due to the increased x-ray tube electron flux, the effective focal spot must have a greater surface dimension than used with standard x-ray techniques. Most carbon based composite materials have a reduced level of absorption to x-ray energies and therefore, the lower KV ranges provide higher radiographic contrast for such evaluations.
The design and make up of the above-noted, known penetrameters, however, are not compatible with the improvements in low kilovolt radiography system resolution because these known penetrameters essentially function as go, no-go gages. With the increased radiographic contrast and sensitivity that very low kilovoltage radiography offers, a more representative penetrameter is needed to judge the adequacy of the imaging technique as well as to serve as an anomaly analysis aid, particularly when conducting a NDE of a composite part having fibers.